I will be able to start my 1st marine tank in the next 3 months. I will be using a Red Sea Max 130D 37 gallon all in one with upgraded skimmer, LED lights. I have questions about a proper RO/DI filter. What GPD should i go with? I would also like the least amount of waste water. Are there units that are more portable then others? Any suggestions would be great.
I recommend 75 gpd models almost exclusively because of their efficiency. For the intial unit, there are a few places in addition to BulkReefSupply you can check out. I highly recommend Buckeye Field Supply for any replacement filters/cartridges, though. They have pleated sediment cartridges that go as low as 0.2 microns, which last a surprisingly long time--but are a bit expensive. I choose to spring for the 0.35 micron pleated filters as a good compromise. Their DI resins also surpass the quality of those offered by BRS. Buckeye offers semiconductor-grade resins as opposed to nuclear grade.
Spectrapure, on the other hand, is probably the go-to place for absolute quality and verified efficiencies, but you'll pay a bit more of a premium for it.
As far as micron sizes go, I agree to go as small as possible, at least on the sediment filters. This will dramatically prolong membrane life and efficiency. My Dow Filmtek membrane is going on 5 years and still outputs 98-99% pure water.